An unknown/withheld number calls my cell phone around once a week. When I answer there is silence. Any help?

The times appear to be random, roughly about once a week. The number always appears as "unknown". There is never a response or any sound at all from the other end - silence (possibly on mute?). The last call i received (tonight), i answered the phone and left it, the caller did not hang up so i... show more The times appear to be random, roughly about once a week.
The number always appears as "unknown".
There is never a response or any sound at all from the other end - silence (possibly on mute?).
The last call i received (tonight), i answered the phone and left it, the caller did not hang up so i terminated the call after more than 5 minutes.
If anyone could give a clue as to whether this is a person, company or machine i would be grateful to you for satisfying my curiosity.
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